Which of the following statements correctly describes the state of things near the top of the descending limb?
(A) The fluid within the descending limb is hypotonic to fluid in the space surrounding the tubule.
(B) The blood within the vasa recta is hypotonic to the filtrate within the descending limb.
(C) The fluid in the area surrounding the tubule is hypertonic to the blood in the vasa recta.
(D) The water in the area surrounding the tubule has a higher water potential than the water in the descending tubule.

The Loop of Henle is a structure within each of the million nephrons within a kidney. As shown in the figure, the two sides have different permeabilities, and there is differential movement across each membrane. The Loop acts as a counter-current multiplier that makes the medulla of the kidney very osmotic. The longer the loop, the higher and more powerful the osmolarity gradient that is created. The gradient is required for the reclamation of water from the urine collecting duct. On the right side of the figure is the urine collecting duct. If the body needs to retain water, anti-diuretic hormone makes this region permeable to water via the introduction of aquaporins, and the osmotic pull of the medulla reclaims the water, out of the collecting duct, which makes the urine more concentrated.

Transcribed Image Text:

Blood vessels Vasa recta Filtrate entering kidney Water flow 1111 Descending limb Osmotic medulla Ions actively pumped Ascending limb Normally impermeable to water Osmotic medulla Collecting Duct Osmotic medulla